Electric submergible pumping systems are widely used throughout the world for moving subterranean fluids to a desired location, e.g. the earth's surface. These electric submergible pumping systems include an electric motor that is drivingly coupled to a pump. Generally, the electric motors used for such applications are rotary motors containing a rotor and a stator. Typically, the rotor lies within a fluid filled cavity within the stator. The fluid not only lubricates the motor, but also cools the motor to prevent overheating.
However, deleterious materials, such as carbon dioxide, hydrogen sulfide and brine, can be found in the subterranean fluids. Those substances can corrode, or otherwise harm components within the electric motors, causing the motor to fail prematurely. Therefore, the cavities within the electric motors are usually filled with uncontaminated motor oil to ensure their long-term successful operation.
The temperature of the motor oil varies as a result of the intermittent operation of the electric motor and the temperature of the fluid surrounding the electric motor. As the temperature of the motor oil rises, for instance, the oil tends to expand and the pressure within the motor tends to increase.
In most submergible pumping systems in use today, this motor oil is partially contained within a device commonly referred to as a motor protector. Motor protectors may serve several different functions. The motor protectors may serve to prevent well fluids and gases from contaminating the motor oil, transmit the torsional power produced by the electric motor to the pump, provide a storage reservoir of motor oil, provide for the expansion and contraction of the motor oil due to changes in temperature, and to equalize the internal pressure of the motor with the pressure of the surrounding subterranean fluids.
Several different approaches have been used to construct motor protectors. One type of motor protector is generally referred to as a "labyrinth" system. Labyrinth systems are exposed at one end to the subterranean fluids and to the electric motor at the other end. These systems retard the migration of subterranean fluid into the electric motor. However, the subterranean fluids can eventually migrate through the labyrinth path to enter the electric motor.
Another approach, referred to as "bladder" or "bag" system, utilizes an elastomeric barrier between the motor oil and the subterranean fluids. However, elastomeric bags suffer from several limitations. First, the repeated expansion and contraction of the elastomeric bag can cause the bag to split or crack under certain conditions. Of course, once an elastomeric bag splits or cracks it no longer protects the motor oil from contaminants which are then free to enter and ultimately damage the motor. Second, elastomeric bags tend to lose their elasticity due to various conditions which may be present in a wellbore. Once an elastomeric bag loses its elasticity, it can no longer expand and contract as needed to satisfy the requirements of the motor oil which it contains. Eventually the bag ruptures, leaving the contaminants free to attack the motor. Third, most elastomers cannot survive in environments where the temperature rises above about 400.degree. F. Above that temperature, most elastomers become brittle causing the bag to break during expansion or contraction. Finally, elastomeric compounds currently used for motor protector bags tend to be relatively permeable as compared to the contaminants within the wellbore fluid. Many wells contain contaminants, such as hydrogen sulfide for instance, which can permeate the motor protector bag and attack the motor. In fact, certain contaminants, such as hydrogen sulfide, also tend to alter the chemistry of some elastomers, causing the elastomers to harden. Once the elastomer has hardened, the bag eventually breaks.
It would be advantageous to have a submergible pumping system that provides the desired functions of a motor protector but without the limitations of the labyrinth or bag systems.